A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth. “Echoes” of that signal are then recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, though, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey. Finally, a 4-D (or time-lapse) survey is one that is recorded over the same area at two or more different times. Obviously, if successive images of the subsurface are compared, any changes that are observed (assuming differences in the source signature, receivers, recorders, ambient noise conditions, etc., are accounted for) will be attributable to changes in the subsurface.
A seismic survey is composed of a very large number of individual seismic recordings or traces. The digital samples in seismic data traces are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Typical trace lengths are 5-16 seconds, which corresponds to 2500-8000 samples at a 2-millisecond interval. Conventionally each trace records one seismic source activation, so there is one trace for each live source location-receiver activation. In some instances, multiple physical sources might be activated simultaneously but the composite source signal will be referred to as a “source” herein, whether generated by one or many physical sources.
In a typical 2-D survey, there will usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces.
Of particular interest for purposes of the instant application are the creation and use in seismic exploration of subsurface models. It has long been known to create best-guess subsurface models based on interpreted seismic data, well logs, etc., and then compare synthetic seismic sections and volumes created from model data with actual recorded seismic data. Then, to the extent that the synthetic data matches the observed data, that would tend to provide confirmation of the correctness of the model and, hence, the correctness of the understanding of the actual subsurface geology. The synthetic data generated from the subsurface models can also be used to design the parameters and methods for acquiring seismic data that best image the structure simulated by the subsurface model. However, specifying the many physical parameters that could potentially be included in such a model can be a daunting task.
Further, the actual subsurface typically contains some number of major rock units (reflectors) together with very large numbers of other layers that are below the limits of normal seismic resolution. These thin layers can act together to modify the seismic signal in ways that are sometimes readily observable and other times not. Creating seismic models that represent this sort of fine scale geology typically requires a degree of knowledge about the subsurface that may not be available until after a well is drilled, at which time seismic modeling may not be necessary for exploration purposes.
Currently most physical property models are either derived from data, or are manually drawn or created with an interactive graphical design program. These methods are limited one or both of two ways: the models only contain features at the scale of the data they are derived from, or they only contain features that are convenient for humans to draw. These types of models do not have the same statistics as true geological layers; simply put, they are often too simplistic.
Heretofore, as is well known in the seismic acquisition and processing arts, there has been a need for a system and method that provides a more efficient method of building subsurface models for use in seismic exploration that does not suffer from the disadvantages of the prior art. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.